Radiofrequency amplifier and method of making and operating the same



Sept. 18, 1928.

A. s. BLATTERMAN RADIO FREQUENCY AMPLIFIER AND METHOD OF MAKING AND OPERATING THE SAIE Filed July 14, 1922 2 ShOQtS-ShBBt 1 do 530 5000 moo an mu 2m aam asau no Pesisfance of Amplifier lnpuf Circa/f I0 Peak/ ance /'n p/a/e circa/7 /00d (ohms).

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I A. s. BLATTERMAN more FREQUENCY AMPLIFIER AND METHOD OF MAKING AND OPERATING was sum Filed July 14. 1922 2 Sheets-Sheet 2 A mpll'flcah'on M z Aw my W "M 3mm h 0 M 5 w y mm m w w m M Ha r n F am M 5% 6 7 w Patented Sept. 18, 1928.

UNITED STATES PATENT OFFlCE'Qf ALBERT IBLATTEBMAN, OF ASIBURY PARK, NEW JERSEY, ASSIGNOB '10 WA.

RADIO, INC., A CORPORATION OF DELAWARE.

nurornnounucr AMPLIFIER Am) marnon or MAKING AND ornm'rme m sum Application filed July 14, 1922. Serial No. 574,881.

pling means in a radio frequency amplifier receiving circuit between the output circuit of a threeelectrode vacuum tube employed as an amplifier therein, and the input circuit of a second three electrode vacuum tube employed as a detector, or as a second amplifier in a multi-stage amplifying receiving circuit.

A furtherobject of this invention is the provisionof a transformer arranged so that the amplifier may be operated to secure high amplification of radio frequency voltages, particularly over a. selected range or ranges of frequencies.

A further object of this invention is to provide within a radio frequency amplifier, a coupling transformer responsive with high efficiency over a relatively wide range of frequencies.

A further object of this invention is the provision, within aradio frequency amplifier, of a coupling transformer, arranged so that the amplifier may be operated to secure high amplification over a selected wide range of frequencies, of simple and economical construction.

Pursuant to my invention the self-inductance of the transformer windings and their mutual inductance on one another are chosen for the particular characteristics of the three electrode vacuum tubes comprised within the amplifier, and for the characteristics of the input circuit of the tube immediately preceding the said transformer, and likewise for the characteristics of the output circuit of the tube immediately succeeding the said transformer.

In the drawings I have shown:

In Figure 1, a circuit diagram of a trans former coupled radio frequency amplifier of conventional type.

- In Figure 2, a diagram of a circuit electrically equivalent of the circuits of Fig. 1.

In Figures 8, 4, 5, 6, 7 curves which will be used in the explanation of the principles of my invention, and

In Figure 8, a side view of a portion of a transformer used in connection with the amplifier of my invention showing the manner in which it is made.

Referring to Fig. 1, 13 and 14 designatetwo three electrode vacuum tubes. The tube 13 receives the radio fre uency voltage to be amplified at its in at e ectrodes, i. e. between its grid 5 and ament 4. The output electrodes of this tube, i. e. its plate 6'and filament 4 are connected to the primary terminals of the coupling transformer 7. The secondary terminals of the transformer 7 are connected to the in ut electrodes of the vacuum tube 14, i. e. its grid electrode 10 and filament electrode 9, respective. The vacuumtube 13 is seen to function as an amplifier tube, and the tube 14 may be a further amplifier tube, as in a multi-stage amplifier, or else a detector. ,1 represents generally the impedance connected to the input electrodes of vacuum tube 13, and 8 represents the gen eralized impedance connected to the output electrodesof the vacuum tube 14. It is to be understood that the impedances 1 and 8 may be constituted in any suitable form. It will be understood that generally speaking it is' desired to secure as high a voltage as possible across the input electrodes, i. e. grid 10 and filament 9 of the tube 14 for a. given voltage originating in the impedance 1. This aim is met by suitablyi designing the coupling transformer 7 according to my invention.

In Fig. 1 are shown in dotted lines the condenser elements 2, 3 and 15 which represent the capacities existing between the several elements of the tube 13, i. e. between the grid 5 and filament 4, the plate 6 and filament 4, and the plate 6 and grid 5 respectively. Similarly the capacities shown in dotted lines at 16, 17, 18 represent the several inter-electrode capacities of tube 14. I have discovered that although these capacities are very small they are nevertheless of great importance in their efi'ect upon the design and operation of an amplifier. The capacities 15 and 18 existing between the grid electrodes and plate electrodes are particularly unique in that they connect the output circuits of the tubes with those on the input side. These capacities thus provide an inherent and unavoidable capacitive coupling from the output or plate circuits of the three electrode vacuum tubes back to the input or grid circuits.

I have discovered that at high radio frequencies, that is atwavelengths less than about 3000 meters, this capacitive coupling between the grid and plate electrodes of a tube permits reaction between the input circuits and the output circuits to such an extent as to require material consideration in designing the amplifier and particularly the coupling transformer 7. Thus, a 'ain referring to Fig. 1, I have discovered that the input impedance 1 and the output impedance 8 both exert important influences in affecting the operation of the transformer 7 on account of their respective reactions on the latter through the inherent tube capacities 15 and 18. As an illustration of this, I have found that a transformer 7'designed to give high amplification in an amplifier circuit in which the impedance 1 is constituted as a tuned circuit with induc tance and capacity in parallel across the grid electrode 5 and filament electrode 1 of the amplifier tube 13, may give practically no amplification at all when the tuned input circuit 1 is changed to a simple untuned inductance coil. In this connection, I have also discovered after a long series of experiments that when the input circuit just referred to is untuned, then in general a larger inductance is required on the primary of the amplifier coupling transformer, than if the input circuit is tuned.

Similarly, I have discovered that if the output impedance 8 comprises for instance a arge condenser, as is usually the case when the vacuum tube 14. is used as a detector, then the amplification of the combination of the tube 13 and transformer 7 at a given frequency is entirely different from that obtained when the impedance 8 is constituted as another transformer similar to 7 as in the case of a multi-stage amplifier when tube 14 is an amplifier .tube in another stage of amplification.

Referring now to Fig. 2, which is representative of circuits electrically and mathematically equivalent to the actual circuits of Fig. 1 in Fig. 2, E represents a source of alternating voltage appearing as the direct result of the signal voltage originating in the impedance 1 of Fig. 1 and impressed upon the input terminals of the amplifier tube 13. 20 and 21 represent respectively the self-inductances of the primary and secondary windings of the coupling transformer 7 of Fig. 1,and 22represents the internal plate-filament resistance of the amplifiervacuum tube 13 of Fig. 1. 23 and 24 indicates respectively an equivalent capacity and resistance circuitally located in proper relation to the other circuit elements and depending for values principally upon the in ter-electrode capacities 2, 3 and 15-of the amplifier tube 13 of Fig. 1, upon the impedance of the input circuit 1 of Fig. 1, and upon the frequency of the impressed radio signal voltage. Similarly 25 and 26 represent respectively an equivalent capacity and resistance Whose values principally depend upon the inter-electrode capacities 16, 17 and 18 of the second vacuum tube 14 of Fig. 1, upon the internal 1 plate-filament resistance of this tube, upon the impedance of the output circuits 8, and u on the frequency. The effective values 0 the capacities 23 and 25 and to a lesser extent of the resistances 24 and 26 are very important in functioning of the coupling transformer 7 and must hence be taken into account in determining the constants of the latter. Accordingly, I determine these quantities by measurement or calculation and correlate them with the constants of the other circuit-elements in the manner hereinafter described, arriving hereby at data for the construction of a coupling transformer 7 providing high amplification over desired ranges of frequencies.

As an example, and in order to indicate the dependance of the values of the effective capacities 23 and 25 of Fig. 2, upon the impedanees 1 and 8 of Fig. 1, I show in Fig. 3 a curve scaled to coordinates indicating measured values of the effective capacity between the grid and filament electrodes of a three electrode vacuum tube with different values of impedance in the output circuit of the tube as represented by 8 of Fig. 1; and on Fig. 4, I show in like manner the variation of the effective capacity between the plate and filament electrodes of a three electrode vacuum tube with variations in the impedance of the input circuit of the tube as indicated at l in Fig. 1.

By properly applying the principles of my invention, I am able to construct radio frequency amplifiers having a variety of characteristics. For example, I am enabled to design transformers of particularly high efficiency in an amplifier circuit and arranged at will to provide high amplification over a very narrow band of wavelengths, or over a relatively wide band of wavelengths, or even over two difi'erent and distinct bands of wavelengths.

In an amplifier of the type under consideration, amplification results by the combined and correlated functioning of the amplifier vacuum tubes 13 and 14: of Fig. 1, the input and output circuits 1 and 8, and the trans former 7; and a measure of the amplification is the ratio of the voltage obtained between the input electrodes 9 and 10 of tube 14, i. e. the second tube, Fig. 1, to the voltage impressed upon the input electrodes 4 and 5 of tube 13, i. e. the first tube.

In order now to explain more clearly the discoveries I have made and their application in the construction of radio frequency amplifiers employing coupling transformers.

I have developed a mathematical solution for the said amplifier circuits indicating the manner in which the various circuit elements combine, and specifying the amplification characteristics of the transformer in such circuit s Such fi fii...

analysis of these circuits was published by'ine in the Wireless World and Radio Review of April 8th, th

and 22nd 1922. Thus, designating V as the voltage across the input electrodes 4 and 5 of vacuum tube 13 (Fig. 1) V as the amplified V L ama-Kat a) R0 K var.

R =The internal plate-filament resistance of vacuum tube 13, 22 (Fig. 2).

In deriving this equation, -I solve the equivalent circuits of Fi 2 and ignore the effective resistance 24 an 26 therein because they are generally very small in comparison with the other circuit impedances and thus do not seriously affect .the mathematical treatment, while their omission greatly facilitates interpretation of the equations.

By referrin to this e nation, it will be apparent that t e ampli cation is maximum when the denominator of the equation is minimum, and the importance of constructing the transformer with suitable values of self and mutual inductances in correlation. with the other circuit constants specified is'thus evident.

I now proceed to an interpretation of the said equation which more clearly indicates the utility and novelty of the discoveries which it formulates. It will first be observed that voltage developed across the input electrodes of vacuum tube 14 and considering the equivalent amplifier circuits of Fig. 2, I arrive at an expression for the amplification as hereinabove defined by the ratio of V, to V,, as follows:

the denominator of the equation comprises two terms. The second term involvesthe internal tube resistance as a factor, and its influence on the amplification, that is the volt age ratio therefore is large when the tube resistance B is large and becomes negligible if R 1s very small. In the latter case the denominator becomes practically and is equal to zero giving the condition for maximum amplification when w that is, at only one frequency. Thismeans that when the circuit constants are such that the second term in the denominator of the equation (1) can be ignored, the transformer will ive high amplification over only a narrow and of. frequencies, and as above described a condition under which this characteristic is developed, is that wherein the internal resistance of the amplifier vacuum tube 13 is low. The curve of Fig. 5 shows the amplification at different frequencies which is obtained under the conditions just discussed, and represents graphically the equation (1) when the second term of the denominator is ignorably small. High amplification is obtained over onl a narrow band of fre quencies. On the ot er hand, if the internal resistance R of thetube 13 is so large that the first term in the denominator can be ignored then the amplification is seen to be maximum when 0, 1 1 2 2 Ii: V 1 1 2 2) 2 1 1 2 2 that is, generally at two frequencies. Practically, this means that high amplification is obtained over two distinct bands of frequen cies, one surrounding the value of w calculated from the last equation using the posllive sign before the second radical therein,

"Gand the other surrounding that value of (0 calculated with the negative sign before this radical. The curve of Fig. 6 gives a. graphical representation of this characteristic.

In practice, neither of the two extreme conditions hereinabove discussed exist, but both terms of the denominator of equation (1) are effective so that generally an amplifier will exhibit peak amplification values at two frequencies, more or less sharply defined according to the relative magnitudes of the two denominator terms.

A qualitative examination of the general amplification characteristic of an amplifier transformer as formulated by equation (1) may be had as follows. The first term of the denominator of equation (1) varies with the frequency somewhat according to the curve of Fig. 7 (for all practical values of coupling between the primary and secondary windings of the transformer, i. e. K l). The second term varies according to the curve 51. The zero value for curve 50, that is, the frequency at which this curve touches the axis of abscissas, is calculable from equation (2), and the similar zero values for curve 51, that is the frequencies at which this curve touches the axis of abscissas is calculable from equation (3). The sum of the curve 50 and the curve 51 is drawnas curve 52, which thus represents graphically the entire term under the radical in the denominator of equation (1). Since the amplification factor is an inverse function of this entire denominator term, peaks of amplification will be obtained where minimum values occur in the aforesaid denominator term and lower amplification is indicated by the higher values of this term. Thus, the curve 52 of Fig. 7 representing the denominator term aforementioned, gives in a general way an inverse representation of the amplification characteristic, the latter having peaks where depressions are shown by curve 52, and depressions where curve 52 shows peaks.

According to the above explanation it will now be clear that the effect of the internal amplifier tube resistance R upon operation of the amplifier transformer and hence upon the design of the same is most important. This resistance, as hereinabove explained tends to accentuate more or less the second term in the denominator of equation (1) this therefore never attains a zero value as it would for zero tube resistance as shown by the curve 51, Fig. 7 above referred to. The peak values of amplification are accordingly reduced but the amplification becomes more nearly uniform over a wide range of frequencies, as evidenced by the representative curve 52 of Fig. 7.

This last characteristic is generally very desirable in practical receiving sets, since it permits greater flexibility in operation and adapts the amplifier to use at various wavetengths.

However, in accordance with the discoveries which I have made and the e nation (1) hereinabove referred to, I am ena led to readily design a transformer responsive over a relatively narrow band of frequencies if such characteristic is desired. For this purpose I have three alternative methods. First,

I employ extremely tight coupling in the transformer (that is K=1) in which case equation (1) shows that maximum amplification is secured at the one frequency corresponding to and diminishes at all frequencies above and below this value. As a second method, I select a winding for the primary of the transformer which will have an inductance of such value as to make the quantity very large as compared with the range of frequencies at which amplification is desired or else I make the secondary inductance of such value that is very small as compared with the frequency at which it is desired to operate. In either case one of the bands of amplification is far removed from the region of operation leaving the amplifier effective throughout a single narrow range of frequencies. As a third method I employ loose coupling in the transformer (K very small) and make L C =L C whereby I obtain an amplification characteristic very sharply responsive to a single frequency.

In a similar way, I am enabled by adjusting the coupling and the inductances of the transformers, to either separate the responsive bands of amplification so as to produce amplification over two widely different and distinct bands of frequencies, or to cause these bands to coalesce giving any desired degree of broadness of amplification response with respect to frequency, all in accordance with the indications of equation I am enabled by the practice of my invention to construct a radio frequency amplifier transformer which is very easy and economical to build and which is particularly efficient over a wide range of frequencies, both of which points are of great practical value. Prior to my said discoveries, radio frequency amplifier transformers were built with iron cores or else their coils were wound with high resistance wire or when usage requirements permitted a transformer responsive over only a narrow band of frequencies copper wire wound coils were employed and wound on a non-magnetic core. A purpose of the iron core construction is to introduce resistance into the transformer circuits through the eddycurrent and hysteresis losses developed in the iron and thus broaden out the tuning effects in the transformer with the purpose of making the transformer responsive over a wide range of frequencies. The insertion of resuitably chosen values of self and mutual inductance of the windings to provide high amplification over whatever range of frequencies may be desired and form these coils of the transformer on a core of non-magnetic material, providing hereby the so called air core construction; Because of the low losses in a transformer of this construction, I secure extremely high efficiency and high amplification, but at the same time, I retain a desirable and satisfactory high amplification over a wide range of frequencies'by correlation of the transformer and other circuit constants, in accordance with my invention. Transformers of the air core type have heretofore had limited application in practice because they were invariably responsive over a narrow frequency range. For exam 1e such a transformer might be found to give hi 'h amplification atsay 300 meters but would be practically valueless below 275 meters and above 325 meters. On the other hand, air core transformers constructed in accordance with my aforesaid discoveries, can readily be arranged to give hi h amplification within a 7 similar range at a wave-lengths from about 160 meters to 500 meters. 1

As an example of the-ap lication of the principles I have discovered and the equa- 1 tions hereinabove disclosed, I have constructed air core transformers for use in connection with present day commercially produced vacuum tubes having inter-electrode capacities of. approximately 4 micro-microfarads and an internal resistance of approximately 30,000 ohms, formed with a primary winding of 143 turns of enameled copper wire and asecondary winding of 220 turns of #40 enameled copper wire, both windings being placed in random fashion in slots or grooves approximately wide cut in an insulating mandrel as shown on Fig. 8. The inside diameter of the slots or grooves is approximate- 1y -1%, their outside diameter approximately and the distance between the slots approximately fi'fi Such transformers give high amplification with vacuum tubes of the characteristics specified at all wavelengths from 160 to 500 meters when used with a substantially tuned input circuit.

Whereas, I have, in the foregoing paragraphs, particularly specified the use of three electrode vacuum tubes for both amplifier and detector stages, it is to be expressly understood that I do not intend to thereby limit myself entirely to such combinations, since the discoveries I have made are obviously applicable also to other types of detectors such as the crystal, the two electrode tube, etc. It will also be understood that the apparatus shown and described herein is shown merely by way of example and that I do not intend to confine myself to the details shown and described nor to the specific use to which the transformers of my invention are applied,

since they may be other purposes.

. What I claim is:

Themethod of making a radio frequency amplifier transformer which consists in adjusting the values of its inductances in relafound useful for many tion to the constants of the circuits with which it is intended to operate according to the following equation:

In J (1 1 2 20 1 q 'V i z a ony whereof, I have signed my I i name to this specification this 10th day of 4' July, 1922. V, ALBERT s. BLATTERMAN. 

